Method for the manufacture of a reflective layer system for back surface mirrors

ABSTRACT

In method for the manufacture of a reflective layer system on a substrate with at least one metallic reflective layer, a dielectric, transparent layer is deposited on the substrate as a silicon oxide containing layer using a suitable PVD process. The coated substrate subsequently is transferred out of the vacuum and at least one metallic reflective layer is deposited via a wet-chemical process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German application no. 10 2011 007500.3 filed on Apr. 15, 2011, and of German application no. 10 2011 080961.9 filed on Aug. 15, 2011, the entire contents of these applicationsbeing hereby incorporated herein.

FIELD OF THE INVENTION

The invention generally refers to a method for the manufacture of areflective layer system for back surface mirrors, which is deposited ona substrate, and which comprises at least one reflective layer as wellas at least one silicon oxide containing layer. The invention refers inparticular to a method for the manufacture of such a layer system forsolar applications.

BACKGROUND OF THE INVENTION

Reflective layer systems have always been used in many different areasof life; however they have become increasingly more important today,e.g. for mirrors that aid the solution concerning energy. While mirrorsfor common interior use, ‘only’ need to reflect the visible componentsof the light spectrum, they must reflect the entire spectrum of sunlight for new solar applications, preferably, wavelengths within a rangeof approximately 300 to approximately 2500 nm.

A reflective layer system comprises a reflective layer or severalreflective layers, which combined contribute to the desired highreflection. For solar applications, this is mostly a combination of asilver and a copper layer, wherein the silver layer faces the incidentlight and the copper layer acts as a protective layer of the silver,however given the layer thickness of the silver does not contribute tothe reflection. Depending on the application, other highly reflectivematerials may be considered such as aluminium, gold, silver, chrome,platinum or molybdenum.

Mirrors fundamentally can be distinguished between front surface andback surface mirrors, depending on which surface of the substrate inrelation to the direction of the incident light produces the mainreflection. Back surface mirrors, therefore, are mirrors that have thereflective coating on the back surface of the substrate facing away fromthe incident light.

The quality of a reflective layer system, in particular regarding solarapplications, is determined also by the value of its Total SolarReflectivity (TSR); that is, its capacity to reflect solar radiation.This value is determined mainly by the reflection capacity of itscoating in addition to the loss of absorption through the substrateitself. For achieving a highest possible reflection, silver mainly as areflective layer and a substrate particularly low in absorption andhighly transparent, e.g. so called white glass or solar glass are used.On the back surface, the silver layer is then finished with a copperlayer, which at the same time serves as an interface layer for apossible subsequent lacquer coating.

The manufacturing process of such reflective layer systems on backsurface mirrors can be described as follows. Subsequent to a suitableprior necessary processing, which can comprise the cutting into arequired shape, the grinding of the edges of the substrate, theirbending and/or tempering of the flat or already bended substrate andother steps, they are, if necessary, polished and washed again. Stillwet, they are then activated through an adhesive-promoting tin chloridesolution. Subsequently, the plate moves through coating stationsone-by-one, where it is coated with silver in a wet-chemical manner, anddirectly following, coated with copper.

Directly following, there can be a coating with a lacquer or withdifferent lacquers of a multi-layered lacquer system. Subsequently, theentire coat is then dried at a temperature of 150° C.-200° C. Throughmanufacturing and drying the lacquer layer, the morphological structureof the reflective layer system is in a way frozen.

Depending on the absorbing characteristics of the substrate and itsthickness, mirrors with a TSR of for instance 93%-94% and a solar glassthickness of 4 mm can be manufactured using the described method. Thisvalue is below the obtainable values which could be determined throughsimulation calculations using corresponding tabulated optical data forsilver.

For improving and specific adjustment of the optical characteristics, areflective layer system frequently can comprise one, mostly severalreflection-enhancing layers that consist of dielectric, low absorptivematerial. Thus, the experts knows of double and multi-layeredalternating layer systems on glass substrates which comprise at leastone series of layers containing a dielectric layer with a highrefractive index, which is facing the incident light, and a transparentdielectric layer with a low refractive index. Because of such afunction, the alternating layer system is arranged on the incident lightfacing side of the reflective layer system. Having a high refractiveindex in regards to solar applications means a material with arefractive index of greater than 2.0, and having a low refractive indexmeans a refractive index of less than 1.8, preferably less than 1.65.

BRIEF SUMMARY OF THE INVENTION

One object of the invention is to present a method for the manufactureof a reflective layer system through which a higher reflection can beachieved in a cost-effective way.

A method is described, which makes use of known and tested wet-chemicalmethods for the deposition of the reflective layer or the reflectivelayers that have very good reflective characteristics, and whichcombines these layers with at least one dielectric, transparent andsilicon oxide containing layer. The latter, in particular, is preferredas a component of the reflective layer system because of its chemicaland mechanical resistance. Additionally, their optical characteristicscan be adjusted very easily via the deposition procedure and/or theirproportion of reactive gas if the deposition takes place using PVD,preferably using sputtering, so that the substrate or an already on thesubstrate deposited stack of layers of several transparent layers iscovered through the use of a silicon oxide containing layer for thedielectric, transparent layer by a material, which has the preferredoptical, chemical and mechanical characteristics comparable to thecharacteristics of the glass substrate.

In addition, the use of a silicon oxide containing layer as anunder-layer proves itself as advantageous, as it presents a cover layer,which can serve temporarily as a preliminary product that is coatedusing PVD, and as it enables the transfer out of vacuum for furthercoating.

Thus, a preliminary product is manufactured, which is very flexiblyapplicable in regard to further processing, so that the followingprocedural steps are largely disconnected from prior steps. The coveringof the uncoated or coated substrate with a silicon oxide containinglayer permits the subsequent wet-chemical deposition, under normalpressure, of differently structured reflective layers or layer systemsas it is known from the direct coating of substrates. In particular, itwas found that a subsequently wet-chemically deposited silver containingreflective layer has good adhesive characteristics in contrast to silverlayers deposited using PVD.

A comparison of TSR values of a reflective layer system manufacturedusing the method according to the invention to a merely wet-chemicallydeposited system has shown better results for the layer system, whichfor its deposition combines PVD and wet-chemical deposition methods.Total Solar Reflectivity values within the range of up to 95% wereobtained.

The method according to the invention permits that both the second andfirst, in a vacuum conducted, portions of the method can largely bevaried and optimized separately. The variation refers, in particular, inthis case to the amount and order of the individual layers or optionalpre-treatments, e.g. the materials used, the addition ofadhesive-promoting layers in several necessary or beneficial locations,or a preferred order of the transparent, dielectric layers. Theoptimization refers, in particular, to the procedural parameters, sothat beneficial or pre-defined characteristics can be adjusted. Forinstance, it is known that the refractive index of the silicon oxide canbe manipulated via the oxygen and nitrogen levels or via the regulationof the procedure.

Also, the method according to the invention permits an intermediatestorage between both basic stages of the method. Depending on thelayers, which are deposited on the silicon oxide containing layer, andthus, depending, in particular, on the reflective layer, variouspre-treatments of the preliminary product can take place. This ispossible prior to and/or following the outward transfer from the vacuumof the preliminary product. For instance, the known chemical activationof the preliminary product can be used prior to the wet-chemicaldeposition, e.g. using a tin chloride solution or another suitablesolution for a subsequent deposition of the silver. Alternatively, andprior to the outward transfer of the substrate, the silicon oxidecontaining layer can be coated with an adhesive layer using PVD methods.Only a small thickness of such an adhesive layer within a range of 5 mnis necessary.

In addition, the use of a silicon oxide containing layer has theadvantage that this layer can be part of a reflection-enhancing,transparent alternating layer system, which comprises, according to oneembodiment of the invention, at least one dielectric layer with a highrefractive index facing the incident light and the silicon oxidecontaining layer as a layer with a low refractive index. Also, othersequences of an alternating layer system with a silicon oxide containinglayer as a finishing layer are possible.

BRIEF DESCRIPTION OF THE DRAWING

In the following, the invention shall be described further using anembodiment. The associated drawing presents an embodiment of areflective layer system of a back surface mirror.

DETAILED DESCRIPTION OF THE INVENTION

The reflective layer system according to the FIGURE comprises asubstrate S, which faces the incident light. The incident light isrepresented by three arrows. For the substrate S, all common materialscan be used, e.g. glass or plastic, also flexible materials.

Directly on a polished, washed and dried substrate made of solar glass,which has the smallest possible absorption, i.e. highest possibletransmission, the following layers are deposited one-by-one throughmagnetron sputtering without any further pre-treatments in a vacuum andof the following thicknesses:

-   -   1.) first adhesive-promoting layer HS, 0.1 nm made of titanium        oxide (TiO₂)    -   2.) dielectric, transparent layer of an alternating layer system        WS with a high refractive index, 40 nm, made of titanium oxide        (TiO₂)    -   3.) dielectric, transparent and silicon oxide containing layer        (SiOS) of the alternating layer system WS, 60 nm, made of        silicon oxide (SiO₂)    -   4.) second adhesive-promoting layer HS, approximately 1 nm        (preferably <1 nm), made of aluminium-doped tin oxide (ZAO)

In the presented embodiments, the deposition of these four layers takesplace always through magnetron sputtering.

The adhesive-promoting layers HS are manufactured either from a ceramictarget with or without an additional inlet of oxygen in DC or MF mode,or from a metallic target in a fully reactive mode with an inlet ofoxygen in MF mode. For a reactive coating from the metallic target in MFmode, the sputtering process is operated in an oxidic mode. As a result,particular intensive plasma combined with small sputtering rates isrealized. For the deposition directly on the substrate S, this resultsin an improved distance of the constantly to the substrate surface boundwater and an optimized realization of a sufficiently thin firstadhesive-promoting layer HS. Additionally, carbonaceous contaminations,which usually have a negative effect on the adhesiveness, are oxidizedto gaseous CO₂, which can be evacuated via the vacuum pump. As long asthe dielectric, transparent layer with a high refractive index of thealternating layer system WS consistent with this embodiment consists ofTiO₂ or of comparably well adhering material, the firstadhesive-promoting layer HS can be omitted.

The layers of the alternating layer system WS are deposited in oneembodiment of the method in a reactive MF mode. As a result, thedielectric layer with a high refractive index is deposited from ametallic target in a fully reactive MF mode with an inlet of oxygen.Alternatively, it can also be deposited from a ceramic target with asmall, additional inlet of oxygen in MF mode.

For a reactive coating from a metallic target in MF mode, the sputteringprocess can be, on the one hand, operated in an oxidic mode.Alternatively, for a reactive coating from a metallic target in MF mode,the sputtering process can be operated within the transitional rangebetween oxidic and metallic range in a so called transition mode in acontrolled manner. This range is characteristic of no or low absorptionat significantly higher coating rates through a suitable choice of anoperating point in comparison to the oxidic mode.

As a result, the dielectric SiO₂-layer with a low refractive index isdeposited either from the metallic target in fully reactive MF mode withan inlet of oxygen, or in voltage regulated transition mode. Theoperating point of the sputtering process is here adjusted using thevoltage of the process and arranged above the inlet of oxygen. In thisway, significantly higher coating rates are obtained at a smallerpartial pressure of the oxygen than in the fully reactive oxidic mode.

The alternating layer system is complemented by a further thinadhesive-promoting layer HS, which is deposited from the ceramic targetwithout or with only a small, additional inlet of oxygen in DC or MFmode. The layer created in this way serves as an adhesive-promotinglayer between the dielectric SiO₂ and the reflective layer R that is tobe deposited subsequently. It is not necessary that this layer has aclosed surface. It can be understood as a so called seed layer. For thisreason, very small layer thicknesses are sufficient in this case. Theyare usually below 5 nm, preferably smaller than 1 nm.

For the deposition of the reflective layer R, the coated substrate istransferred out of the vacuum subsequently, and using a wet-chemicalmethod, the following layers are deposited one-by-one:

-   -   5.) metallic, reflective layer R made of silver (Ag);    -   6.) metallic, reflective functional layer F made of copper (Cu).

The light incidence takes place in the Figure through the substrate S,so that the metallic, reflective layer R is facing the light incidencein comparison to the metallic, reflective functional layer F.

The layer system according to the Figure is coated on the side facingaway from the incident light with a lacquer, which, in the embodiment,has three lacquer layers L1, L2, L3, outside the vacuum system, andsubsequently is dried. Alternatively, individual lacquer layers or otherlacquer systems are possible too.

In the described embodiment, a pre-treated surface O of the substrate isproduced through the deposition of the first adhesive-promoting layerHS. A pre-treatment of the surface of the substrate S that is to becoated can take place alternatively also through a direct current (DC)or medium frequency (MF) glow discharge, which mostly is ignited in ararefied gas atmosphere, that can contain Ar, O₂, N2, CDA (CompressedDry Air) or any combination of these, at a pressure of 2-5×10⁻².

The first adhesive-promoting layer HS or a pre-treatment can also beomitted, so that the silicon oxide containing layer or the alternatinglayer system as described in the above embodiment are directly depositedon the substrate S. This is possible, for instance if the first layerthat is to be deposited on the substrate S of the alternating layersystem WS is made of titanium oxide or a comparably well adheringmaterial.

Also, the second adhesive-promoting layer HS is optional as the siliconoxide containing layer SiOS that is to finish the alternating layersystem WS has a good mechanical and chemical resistance, and thus, issuitable for the subsequent outward transfer and further treatment ofthe substrate taking place in atmospheric conditions.

Another embodiment therefore can have the following layer structure ofthe reflective layer system on a washed glass as the substrate:

-   -   1.) dielectric, transparent layer with a high refractive index        of an alternating layer system WS made of titanium oxide (TiO₂)    -   2.) dielectric, transparent and silicon oxide containing layer        with a low refractive index (SiOS) of the reflective layer        system WS made of silicon oxide (SiO₂)    -   3.) adhesive layer HS made of an aluminium-doped tin oxide (ZAO)    -   4.) metallic reflective layer R made of silver (Ag);    -   5.) metallic, reflective functional layer F made of copper (Cu)    -   6.) triple-layered lacquer L1-L3.

Also, the adhesive-promoting layer HS can be omitted in this reflectivelayer system.

Alternatively, a chemical activation of the surface of the silicon oxidecontaining layer SiOS through the known method using a solution actingas an adhesive agent, e.g. tin chloride, can take place as it is knownfrom purely wet-chemically executed methods.

As a result for instance, the following layer structure of thereflective layer system on a washed glass as the substrate is obtained:

-   -   1.) dielectric, transparent layer with a high refractive index        of an alternating layer system WS made of titanium oxide (TiO₂)    -   2.) dielectric, transparent and silicon oxide containing layer        with a low refractive index (SiOS) of the reflective layer        system WS made of silicon oxide (SiO₂)    -   3.) first adhesive layer HS made of an aluminium-doped tin oxide        (ZAO)    -   4.) second adhesive layer HS through a chemical activation with        tin chloride    -   5.) metallic reflective layer R made of silver (Ag);    -   6.) metallic, reflective functional layer F made of copper (Cu)    -   7.) triple-layered lacquer L1-L3.

The ZAO adhesive-promoting layer HS can be omitted also in thisreflective layer system, in this case the first adhesive-promoting layerHS.

For each of the above described optional layer systems, furtherprocessing is possible using wet-chemical depositing of one or severalsubsequent reflective layers in atmospheric conditions. The processingcan follow directly after the outward transfer from vacuum or after anystorage of the coated substrates S.

Also, the materials used for the reflective layer R and the reflective,functional layer F can deviate from the silver or copper as stated inthis case. For the reflective layer R, other metals can be used such asaluminium, gold, platinum or an alloy, which contains at least one ofthe mentioned metals. The mentioned metals all have a comparably high,in particular, solar reflection, if necessary for certain wavelengthssuch as for gold and platinum, and thus, are suitable for the reflectivelayer system.

For the metallic, reflective functional layer F, materials such ascopper, nickel, chrome, stainless steel, silicon, tin, zinc or an alloy,which contains at least one of the mentioned metals, are considered.Through these materials, the reflective characteristics can be combinedwith mechanical and/or chemical protection.

For other solar applications, other reflective materials are consideredwithout having a direct influence on prior treatment and coatingprocesses.

Also, for the dielectric layer with a high refractive index of thealternating layer system, various materials can be used, e.g. alsoniobium oxide (Nb₂O₅).

For the adhesive-promoting layer HS, other materials can be used as analternative, e.g. materials made of a group of oxides comprising ZnOx,SiOx, SnOx, TiOx or ZrOx, wherein x≦2.

Depending on the quality of the glass (absorption) and thickness,mirrors can be manufactured with a TSR according to ISO 9050:2003 of upto 95% using the layer system according to the invention, e.g. accordingto the above embodiments and the method of its manufacture for instancewith a solar glass thickness of 4 mm.

1. Method for manufacture of a reflective layer system for a backsurface mirror on a substrate comprising at least one transparent,dielectric layer and at least one metallic reflective layer, comprising:deposition of a dielectric, transparent layer containing silicon oxideon a surface of a substrate by physical vapor deposition in a vacuum toprovide a coated substrate; subsequent transfer of the coated substrateout of the vacuum, and deposition of at least one metallic reflectivelayer upon the dielectric, transparent layer of the coated substrateusing a wet-chemical process.
 2. Method for the manufacture of areflective layer system according to claim 1, wherein a surface of thelayer containing silicon oxide, following the transfer of the coatedsubstrate and prior to the deposition of the at least one reflectivelayer, is chemically activated.
 3. Method for the manufacture of areflective layer system according to claim 1, wherein the layercontaining silicon oxide, prior to the transfer of the substrate iscoated with an adhesive-promoting layer with a thickness of less than 5nm using a PVD process.
 4. Method for the manufacture of a reflectivelayer system according to claim 1, wherein the surface of the substrateis pre-treated and then coated using plasma treatment and/or temperingin a vacuum.
 5. Method for the manufacture of a reflective layer systemaccording to claim 1, wherein the surface of the substrate ispre-treated and then coated using deposition of an adhesive-promotinglayer with a thickness in a range of less than 5 nm.
 6. Method for themanufacture of a reflective layer system according to claim 1, whereinan alternating layer system is deposited on the substrate using physicalvapor deposition, which comprises at least one series of layers with onedielectric layer having a high refractive index and one dielectric layerhaving a low refractive index, wherein the silicon oxide containinglayer comprises the one dielectric layer having a low refractive indexand finishes the side of the alternating layer system facing away fromthe substrate.
 7. Method for the manufacture of a reflective layersystem according to claim 1, wherein following deposition of the atleast one reflective layer, a metallic, reflective functional layer isdeposited in a wet-chemical process on the at least one reflectivelayer.